Urinary tract obstruction or leakage of urine will usually result in azotaemia, hyperkalaemia, acidosis and often hypovolaemia. Hypovolaemic animals need volume resuscitation with an isotonic crystalloid to correct hypoperfusion. Fluid boluses appropriate to the degree of hypovolaemia present should be administered, and any further requirements determined by response to treatment.
Hyperkalaemia can be life-threatening. The rising potassium levels have an effect on the myocardium and cause cardiac arrhythmias; animals are often recumbent and semi-conscious. If a bradyarrhythmia is detected, an electrocardiogram (ECG) should be performed (see Chapter 10). Correcting hypoperfusion and establishing urine drainage will reduce potassium levels, but often additional treatment is required. This is especially the case where sedation to relieve an obstruction and place a urinary catheter is required. The cardiotoxic effects of hyperkalaemia greatly increase anaesthetic risks. Calcium gluconate 10% solution administered slowly by intravenous injection (0.5–1.5 ml/kg) is very useful. While it has no effect on serum potassium levels, it stabilises the threshold potential of the myocardial cells; the effect lasts for 20–30 minutes. For cases where the hyperkalaemia is likely to be ongoing (e.g. in urinary tract trauma or acute renal failure), intravenous neutral insulin and glucose can be administered. The resulting uptake of glucose and potassium into cells reduces serum potassium concentrations.
Urinary Tract Obstruction
In most cases urinary obstruction occurs at the level of the bladder or urethra, although occasionally a ureter can become blocked. The obstruction is most commonly due to uroliths (see Figure 12.2), or urethral plugs in cats, although neoplasia and granulomatous lesions can also cause a blockage. Cats with feline lower urinary tract disease may present in a similar fashion, whilst there may be no physical obstruction, the pain and muscle spasm associated with the condition may lead to a ‘functional’ obstruction.
Owners often report stranguria, dysuria or anuria. Females are less commonly presented than males, due to having wider, shorter urethras. The clinical condition of the patient depends on the duration of obstruction and whether the obstruction is complete or partial (the animal being able to pass some urine). Although the animal will be showing discomfort, systemic signs may not be apparent in the first 24 hours until azotaemia develops. Careful palpation of the abdomen will usually reveal a large tense bladder; care must be taken not to cause rupture.
Following stabilisation, sedation or anaesthesia is usually necessary to enable a urinary catheter to be passed and the obstruction relieved. As with all critical animals, the minimum possible dose of sedative or anaesthetic agent should be used. Agents are selected to minimise the effect on the cardiovascular system.
In male dogs, uroliths most commonly lodge at the narrowing of the urethra just proximal to the os penis. Most uroliths lodged in this position can be ‘hydropulsed’ (flushed with saline) back into the bladder and removed surgically via a cystotomy later (see Figure 12.3). Once the obstruction has been hydropulsed the catheter can be advanced into the bladder to drain it. If the obstruction cannot be dislodged, attempts can be made to pass a very narrow catheter past the obstruction into the urinary bladder as a temporary measure, but usually a pre-scrotal urethrostomy is required to remove the stone surgically.
Relief of obstruction in male cats requires urethral catheterisation. Some cases may have an intra-penile obstruction; usually the penis will look cyanotic, and it may be possible to break down the obstruction manually with massage. For catheterisation, a 3 French catheter with open tip is used, and saline flushed through the catheter as it is advanced (see Practical techniques at the end of the chapter). In some cases where catheterisation proves difficult, emptying the bladder via cystocentesis can ease catheter placement, as the full bladder is no longer pushing caudally on the urethra. If cystocentesis is necessary, then it should be performed with as narrow a gauge needle as possible to minimise damage to the bladder wall and reduce the risk of urine leakage (see Figure 12.5). If the obstruction cannot be cleared, a tube cystostomy may be necessary (see ‘Urinary diversion’).
Where a urinary catheter is to be left in place (indwelling), it should be connected to a closed collection system to prevent ascending infection (see Urine collection systems). Where an indwelling catheter is to be placed it is important to minimise discomfort, and trauma from the presence of the catheter. Foley catheters are suitable in the dog and bitch (a stylet is helpful to assist in placing them) as they are soft, and can sit in the bladder neck rather than having a long length of catheter in the bladder. Once the balloon has been filled with saline, the catheter can be pulled caudally to seat the balloon in the bladder neck; this prevents removal of the catheter.
Urinary Tract Trauma
Trauma can result in injury to the urinary tract anywhere along its length, with the potential for leakage of urine into the surrounding area. Blunt trauma (such as road traffic accidents, kicks or falls) is the most common reason for damage to the urinary tract, but penetrating trauma (bite wounds, ballistic injuries) or trauma secondary to obstruction may also be seen. Leakage of urine from the kidney or the majority of the length of the ureter will lead to accumulation of urine in the retroperitoneal space whereas leakage from the distal ureter, urinary bladder or the proximal urethra will result in uroperitoneum as the fluid fills the abdomen. Leakage from the more distal urethra results in urine accumulating in the tissues of the perineal area, causing inflammation and often sloughing of the skin.
Urine leakage into the retroperitoneal space from the kidney or ureter may be difficult to diagnose; the fluid does not enter the abdomen, so cannot be detected by abdominocentesis. Plain radiographs may show an increase in the size or a change in density (seen as ‘streaking’ due to the different radiographic densities of fat and fluid) of the retroperitoneal space (see Figure 12.7). Diagnosis is assisted by the use of iodine contrast agents to perform excretory urography. Damaged kidneys may require partial or complete nephrectomy. Ureter trauma may be débrided and repaired with anastomosis or implantation of the end of the ureter into the bladder.
Rupture of the urinary bladder is commonly associated with blunt trauma to the abdomen. Large deficits in the bladder wall will result in the rapid loss of urine into the abdomen and rapid onset of clinical signs, but smaller leaks may take several days to produce recognisable symptoms. The presence of urine in the abdomen initiates a chemical peritonitis, and being hyperosmolar, the urine draws water from the extracellular space into the abdomen, causing dehydration. The rapid equilibrium of electrolytes across the peritoneal membrane results in hyperkalaemia and acidosis. Diagnosis is usually made by abdominocentesis (see Chapter 11). If the abdominal fluid has an increased creatinine level compared to blood creatinine concentrations, it is suggestive of uroabdomen (creatinine is a relatively large molecule, and as such does not equilibriate across the peritoneal membrane, whereas urea – being smaller – rapidly crosses). Abdominal ultrasound and retrograde urethrocystograms may also be useful in making a diagnosis of bladder damage.
Animals that have had a uroabdomen for some time will require stabilisation prior to exploratory surgery and repair. The goals of stabilisation are to correct fluid deficits and electrolyte imbalances by a combination of fluid therapy and urine drainage. Dehydration and hypovolaemia should be corrected with intravenous isotonic crystalloid fluids. Drainage of urine from the abdomen can be managed by abdominocentesis, passing a transurethral catheter into the abdomen, or by placing a fenestrated abdominal drain under local anaesthesia (see Practical techniques at the end of the chapter). Urine collection must be into a ‘closed’ system.
Urethral damage can result from fractures of the pubis, bite wounds to the perineum, penetrating wounds in the pelvic area or secondary to obstruction and traumatic catheterisation. Retrograde urethrography is usually necessary to confirm the diagnosis (see Figure 12.8). Diversion of urine via a tube cystostomy may be required to allow healing of the urethra.